1. Field of the Invention
The present invention relates generally to circuitry for regulating power supplies. In particular, the present invention relates to a regulator system with a circuit to regulate the output of a power supply at low cost and high efficiency with a simple and compact design.
2. The Prior Art
A common problem with audio amplifiers that have conventional capacitor and choke input linear power supplies is electrical as well as thermal overstress on amplifier circuitry when there is excessively high voltage present on the alternating-current (AC) line or when the amplifier is producing low power output. The high AC line voltage can place undue stress on the amplifier and power supply components and can push them beyond their recommended safe operating conditions. To provide adequate safety margins, the amplifier and power supply circuitry have to be overbuilt to tolerate these conditions, which leads to higher component costs. Other system configurations such as high voltage 70V and 100V distributed audio systems require that maximum audio signal voltages be limited to avoid problems with their respective speaker loads.
Conventional approaches to voltage regulation include linear and switching regulators. For example, U.S. Pat. No. 6,072,708 discloses a phase controlled switching regulator power supply and U.S. Pat. No. 6,020,724 discloses a regulated capacitor charging circuit using a high-reactance transformer. These approaches have problems with complexity, efficiency, cost and the generation of electrical interference.
The present invention provides a regulator system including a simple circuit with a low component count and a low cost of implementation. The system includes an electrical system comprising a power supply, preferably a capacitive input power supply, for inputting an input alternating-current waveform having a peak voltage, preferably on the secondary winding of a transformer coupled to the power supply, and a phase modulator synchronous rectifier circuit, such as a full-wave diode bridge and a plurality of opto isolated power triacs connected in series with the bridge, coupled to the power supply.
The regulator system includes a ramp generator, preferably comprising a storage capacitor and a current source, which is coupled to the electrical system and which generates a linear ramp cycle between a minimum and a maximum voltage. A zero crossing detector connected to the ramp generator detects zero crossings of the input alternating-current waveform and synchronizes the linear ramp cycle of the ramp generator with the input alternating-current waveform.
A peak voltage detector connected to the ramp generator stores the maximum voltage reached by the generator. A resistive divider connected to the peak voltage detector reduces the peak detector voltage to a scaled voltage value corresponding to a position in time on the input alternating-current waveform below the peak voltage.
A timing window comparator connected to the ramp generator and the resistive divider compares the scaled voltage value with the instantaneous value of the ramp voltage to establish a time window signal.
The regulator system also includes a conduction angle ramp generator connected to the timing window comparator. The conduction angle ramp generator, which preferably comprises a voltage controlled current source and a timing comparator, generates a conduction ramp cycle between a maximum and a minimum conduction ramp voltage. The conduction ramp cycle is synchronized with the time window signal and the zero crossing detector so that the maximum conduction ramp voltage starts at the beginning of the time window signal and the minimum conduction ramp voltage occurs at the next zero crossing of the input alternating-current waveform.
An error amplifier connected to the power supply averages the output voltage on the power supply to an average output voltage and compares the average output voltage to a reference voltage to generate an error voltage.
An output comparator is connected to the conduction angle ramp generator and the error amplifier. The output comparator compares the error voltage to the value of the conduction ramp voltage to generate an output comparator output. The output comparator output is connected to the rectifier circuit to activate the rectifier circuit at selected times during the input alternating-current waveform.
In another aspect, the present invention provides a method of regulating the output of a power supply. An electrical system is provided including a power supply such as a capacitive input filter power supply, for inputting an alternating-current waveform having a peak voltage, preferably on the secondary winding of a transformer coupled to the power supply, and a phase modulated synchronous rectifier circuit, such as a full-wave diode bridge and a plurality of opto isolated power triacs connected in series with the bridge, coupled to the power supply.
The input alternating-current waveform is synchronized with a linear ramp cycle extending between a minimum and a maximum ramp voltage. A time window signal is established corresponding to a position in time on the input alternating-current waveform below the peak voltage band on the linear ramp cycle.
The time window signal is synchronized with the linear ramp cycle and a conduction ramp cycle extending between a maximum and a minimum conduction ramp voltage such that the maximum conduction ramp voltage starts at the beginning of the time window signal and the minimum conduction ramp voltage occurs at the next zero crossing of the input alternating-current waveform.
The value of the conduction ramp voltage is compared to an error voltage generated by a comparison between the average output voltage on the power supply and a reference voltage. The rectifier circuit, preferably the opto isolated power triacs of the circuit, is activated at selected times during the input alternating current based on this comparison.
The present invention is an improvement over prior power supply regulation approaches and may be applied for use in an audio amplifier to increase its reliability under high line conditions and, if desired, limit the amplifier""s maximum output voltage swing. The present invention can also be used to regulate devices other than audio amplifiers and devices that employ capacitive input filtering. Because of its low component count, the regulator system is physically compact and requires little in the way of printed circuit board (PCB) space and mechanical packaging. Only the output triacs need to be mounted on heat sinks to ensure circuit reliability.
The regulator system also does not generate any appreciable electrical noise, as it contains no high frequency switching elements. The efficiency of this approach is enhanced by the absence of dissipative elements in the design to waste power. The control circuit makes a decision about the conduction timing of the rectifier circuit based upon the load on the power supply and replenishes the charge on the main filter capacitors that was lost on the last one half AC line cycle. The regulator system is also self-adjusting for changes in line frequency and is applicable to all world line frequency standards. It also reduces the AC inrush current of the power supply as it gradually ramps up the charge on the main filter capacitors over several AC input line cycles.